Histological quantitative study on the formation of stress fibers (SF) and expression of heat shock protein (HSP) in endothelial cells in response to mechanical stretching was performed using excised rat arterial beds. They were classified into two groups in respect of inductivity of these parameters. In order to examine the effects of such stress on the functions of endothelial cells, histochemical detection of intracellular nitric oxide (NO) in fixed tissues was attempted. We found that a combination of aldehyde fixatives with 4, 5-diaminoflurescein diacetate is useful for this purpose. Using this NO detecting system, NO production activity in endothelial cells in vivo will be examined in near future study after loading some kinds of stresses.On the other hand, the relationship between SF formation and HSP expression in endothelial cells loaded mechanical stretching was also analyzed. While inductivity of HSP25 and HSP90 in the cells in response to the stress was relatively low, HSP70 expression was evidently stimulated in the cells applied the stress. Furthermore, it has been demonstrated that the inhibition of HSP70 expression by treatment of the cells with quercetin resulted in suppression of SF formation. However, inhibition of SF formation by ttreatment of the cells with cytochalasin D had no effect on HSP70 expression. Thus, HSP70 probably acts as a molecular chaperone in SF formation in endothelilal cells in response to mechanical stretching.